1. Field of the Invention
This invention relates to a heating device including a heat applying member heated by a heat source and a pressure applying member pressure contacting the heat member with a prescribed pressure for fixing a toner image onto a printing sheet by means of heat and pressure, a fixing apparatus including the heating device, and an image forming system, such as a copier, a printer, a facsimile, etc., including the fixing apparatus.
2. Discussion of the Background Art
A conventional fixing apparatus mounted on an image forming system performs fixing by applying heat and pressure to a printing medium fed between a heat applying member and a pressure applying member while pressurizing melted developer (toner) onto the printing medium.
Such a heat applying member is formed in a roller or belt state and installs a heat source, such as a halogen heater, an IH coil, etc. Otherwise, the heat source is installed in a roller that winds the belt state heat-applying member or is arranged in a circumferential section of the heat-applying member.
In this type of the fixing apparatus, a start up operation for heating the heat-applying member up to a temperature capable of fixing toner is needed as a warm up operation. The start up time is preferably short for a user, and is sometimes described in a brochure as a specification of a product.
Conventionally, a temperature priority mode is adopted, in which a start up operation is regarded as completed when temperature of the heat-applying member reaches a prescribed setting level.
However, since a time taken by the start up operation varies depending upon unevenness of parts of the heat-applying source in the temperature priority mode, the user is dissatisfied by the mode
Conventional temperature and heat application control technologies are described in Japanese Patent Registration No. 3350315, and Japanese Patent Application Laid Open Nos. 2005-345989 and 62-70886. Specifically, a time priority mode is adopted in Japanese Patent Application Laid Open No. 62-70886, in which a start up operation is regarded as being completed when a prescribed time period has elapsed after a start up in order to make a start up time period constant.
However, the time priory mode can affect an image quality depending on a condition where a fixing apparatus is used. Specifically, since calorie supplied from a heat source per hour decreases when an input voltage thereto is low, temperature of the heat applying member decreases, so that a fixing error, such as a cold offset, etc., occurs. Further, calorie necessary to fix developer (toner) onto a printing medium varies depending on a condition such as ambient temperature.
Accordingly, it is hard to determine completion of a start up operation for the heat-applying member only based on the elapsing time from the starting up of heat application.